It is known in the art relating to heating to provide homogenous heating of materials which consist of organic or mixtures of organic and inorganic materials. The need for such heating is great. The materials may be made up of solid, or liquid as well as a mixture of solid and liquid components. The materials to be heated may be of both large and small volumes. For example, the preparation of foodstuff in the food industry, the cooking of food in restaurants and homes, the sterilization of offal, the rotting of wood fibers and the degradation drying and sterilization of sludge.
There are applications where a fast heating without appearance of hot spots or hot areas is a necessity. One example of such an application is the heating of cold blood in connection with blood transfusions in medical care.
In industrial processes such as the drying of wood, there is a need to measure and utilize changes in dielectric properties of the materials being heated.
Established heating techniques, such as heating with microwaves, heating with conventional heat radiation and heat convection commonly have heat absorption in a load that is characterized by little or no depth of penetration. The heating of the inner parts of a load will be done by heat transmission from the heat absorbing surfaces. In the most common dielectric materials, heat transmission is a slow process. As a consequence, large volumes of organic materials require a long time to achieve a homogenous heat distribution.
It is also previously known that dielectric materials can be heated by oscillating high frequency electric fields generated between a pair or pairs of electrodes. This technology has the disadvantage of not being flexible to accommodate differences in load geometry and load composition.
It is also known that it is possible to heat a dielectric load by emitting electromagnetic radiation from an antenna into a cavity with walls made of electrically conducting material.
Heating with microwaves in a resonant cavity has been established for many years. A resonant cavity has the advantage of making possible an even distribution of microwaves.
A resonant cavity of a microwave oven requires that the following certain physical conditions be met:
a. The infinite conductivity of the walls.
At frequencies above 900 MHz (micro wave frequencies), the currents in a cavity wall are concentrated at the surface. An approximation according to Maxwell's equations valid for the infinite conductivity of walls results in a negligible error. With decreasing frequency the skin depth increases. At frequencies below 300 MHz, the skin depth is so considerable that according to known technology a resonant cavity is not regarded as possible.
b. The design of the cavity.
The cavity walls shall have approximately infinite conductivity and the cavity shall meet required dimensions for a resonant cavity. For example, at least one length of one side of a rectangular cavity shall not be below half a wavelength. To obtain a resonant circular cavity, the diameter shall correspond to 0.76% of a wavelength. It is possible to obtain a homogenous distribution of microwaves in a cavity. However, microwaves have an insignificant depth of penetration. Thus, as to many different applications, particularly if the loads are thick, the result will be a superficial and inhomogeneous heating.
Heating in a resonant cavity at frequencies below 300 MHz has theoretically, as to many applications, considerable advantages for getting a fast and homogenous heating of large loads of dielectric materials. Due to reasons mentioned above, such cavity has been regarded as an impossibility.